Nitrogen oxides are included in various exhaust gases, and the nitrogen oxides can be not only the cause for generating photochemical smog and acid rain, but also directly adversely affect a human body. Therefore, various means have been conventionally proposed for eliminating nitrogen oxides in exhaust gases. In particular, a method wherein a nitrogen oxide-containing exhaust gas is contacted with a catalyst to reduce the nitrogen oxide has been already practically utilized in some field.
Examples of the method include (a) a method for treating nitrogen oxides in an exhaust gas from a gasoline engine of an automobile with a three-way catalyst, and (b) a method of a selectively catalytic reduction of nitrogen oxides in an exhaust gas from large scale facilities such as a boiler with ammonia as a reducing agent. Furthermore, recently there has been proposed (c) a method for treating nitrogen oxide-containing exhaust gases using a hydrocarbon as a reducing agent in the presence of a catalyst comprising a metal such as copper having supported thereon a metallic oxide such as alumina or a catalyst comprising various metals having supported thereon a zeolite (JP-A-63-100929 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")).
In the above-described method (a), hydrocarbon components and carbon monoxide included in a combustion exhaust gas from a gasoline engine of an automobile are converted to water and carbon dioxide with a catalyst comprising a metal belonging to the platinum group and, at the same time, nitrogen oxides included in the exhaust gas are reduced to nitrogen. An exhaust gas is required to comprise an appropriate amount of oxygen so that nitrogen oxides may be effectively reduced under the reaction conditions. Thus, this method has a problem in that it cannot be used in principle in an atmosphere in which an excess amount of oxygen is present as in a diesel engine.
The method (b) uses ammonia as a reducing agent which is very toxic and often must be handled as a high pressure gas so that it cannot be easily conducted. Furthermore, since the facilities are inevitably have a large size, it is technically difficult to be applied to a small size-exhaust gas generation source, especially a moving generation source. Thus, it is also undesirable in an economical aspect.
In the method (c), as in the above-described method (a), a main subject to be treated is a combustion exhaust gas from a gasoline engine of an automobile. This method is difficult to be applied to a treatment of exhaust gases from a diesel engine and the durability of the catalyst is still insufficient. That is, the above-described catalyst comprising a carrier such as alumina or a zeolite having supported thereon a metal such as copper is readily impaired with a sulfur oxide discharged from a diesel engine and the decrease in activity of the catalyst may occur by the aggregation of metals which are active species.
Accordingly, it is earnestly desired to develop a method capable of purifying an exhaust gas by effectively reducing nitrogen oxides in the exhaust gas in the presence of an excess amount of oxygen and even in a case where the gas contains water vapor or a sulfur-oxide.